Non-woven fabrics manufactured from a low-melting-point resin as the sheath component and a high-melting-point resin as the core component have been well received for their properties such as feeling (touch) and non-woven tenacity, and have widely been used as the surface materials for hygienic products such as paper diapers and sanitary napkins. Such non-woven fabrics are typically manufactured by processing a heat-fusible composite fiber into a web through carding or air-flow opening, then melting the sheath component by heat and pressure, and bonding fiber intermingling points.
Processes forbonding fiber intermingling points are roughly divided into the heat-pressure method using heat embossing rolls, and the hot-air bonding method using a suction band dryer or a suction drum dryer. Non-woven fabrics manufactured by these methods are called point-bonded non-woven fabrics and through-air non-woven fabrics, respectively, and are used according to their applications.
Such fibers known as heat-fusible composite fibers include, for example, a composite fiber consisting of a high-density polyethylene sheath component and a polypropylene core component (hereafter referred to as HDPE/PP-based heat fusible composite fiber), and a composite fiber consisting of a high-density polyethylene sheath component and a polyester core component (hereafter referred to as HDPE/PET-based heat fusible composite fiber). Also included is a composite fiber consisting of a propylene-based copolymer sheath component and a polypropylene core component (hereafter referred to as co-PP/PP-based heat fusible composite fiber) as disclosed in Japanese Patent Publication No. 55-26203, and Japanese Patent Application Laid-open Nos. 4-281014 and 5-9809.
Among these fibers, since the co-PP/PP-based heat fusible composite fiber has propylene components in both resins constituting the sheath and those constituting the core, strong affinity exists between the sheath and core components, and, in contrast to HDPE/PP-based or HDPE/PET-based heat fusible composite fibers, the sheath and the core are not prone to delamination. In addition, since, relative to HDPE, co-PP in the sheath component excels in the ability of heat-sealing with other resins, non-woven fabrics produced from the co-PP/PP-based heat fusible composite fiber are highly evaluated for their high strength when processed into paper diapers or hygienic products together with non-woven fabrics or films produced from other resins.
When a non-woven fabric is produced from a heat fusible composite fiber, the feeling (touch) of the non-woven fabric is incompatible with its tenacity. Conventionally, since non-woven fabrics for hygienic materials are required to have a sufficient tenacity and as fast a production speed as possible, they have often been produced through heat treatment at a relatively high temperature. As are cent trend, however, softer feeling (touch) has been demanded in non-woven fabrics used as the material of hygienic products. Therefore, a lower temperature is often employed for the heat treatment of non-woven fabrics produced from co-PP/PP-based heat fusible composite fibers, resulting in a problem of a lower tenacity of the non-woven fabrics.
For this reason, the development of co-PP/PP-based heat fusible composite fibers is required for producing non-woven fabrics which satisfy the two incompatible demands for high tenacity and soft feeling (touch).
In existing co-PP/PP-based heat fusible composite fibers, however, the difference in melting points between resins used as the materials for the sheath and core components is smaller than that of HDPE/PP-based or HDPE/PET-based heat fusible composite fibers. In addition, orientation and crystallization of the resins occur during the spinning and drawing processes, further decreasing the difference in melting points of the two components. If the heat treatment temperature is raised to attain tenacity sufficient for the non-woven fabric to be used as the surface material of hygienic products, feeling (touch) is degraded and dimensional stability is lowered, raising problems. For example, in point-bonded non-woven fabrics feeling will become hard, and in through-air non-woven fabrics thickness will decrease, bulk will lower, and dimensional stability will lower due to heat shrinkage.
An object of the present invention is to provide a heat-fusible composite fiber which enables the fabrication of non-woven fabrics having high tenacity and excellent feeling (touch) with high dimensional stability, and to provide a non-woven fabric produced by the heat-treatment of said fiber through methods such as heat-and-pressure bonding or hot-air bonding.